A printed circuit board widely used in an electronic implement comprises, for example, a synthetic resin sheet and a sheet of copper foil bonded onto each of the both surfaces of the synthetic resin sheet through a glass fiber sheet impregnated with an epoxy resin (hereinafter referred to as the "prepreg").
A typical method for manufacturing a printed circuit board is described with reference to FIG. 1. FIG. 1 is a schematic descriptive view illustrating a method for simultaneously manufacturing a plurality of printed circuit boards by the use of a hot press.
First, a sheet of copper foil 10a, a prepreg 9a, a synthetic resin sheet 8, another prepreg 9b and another sheet of copper foil 10b are placed one on the top of the other in this order to prepare a set of laminated sheets 7a. Another set of laminated sheets 7b is prepared in the same manner as described above, and thus a plurality of sets of laminated sheets 7a and 7b are prepared.
The above-mentioned plurality of sets of laminated sheets 7a and 7b are arranged between a pair of pressing portions of a hot press 13, as shown in FIG. 1, i.e., between a lower pressing portion 15 provided on a stand 14, and an upper pressing portion 16 arranged upward the lower pressing portion 15 at a prescribed distance therebetween so as to face same, and are pressed at a prescribed temperature. For the purpose of uniformly pressing each of the plurality of sets of laminated sheets 7a and 7b, there are employed a pair of pressing plates made of steel, i.e., a lower pressing plated 11a and an upper pressing plated 11b, each having a thickness of about 5 to 10 mm, and a plurality of separating sheets made of steel, i.e., a lower separating sheet 12a, an intermediate separating sheet 12b and a lower separating sheet 12c, each having a thickness of about 1 to 2 mm.
A block is thus prepared by placing, from down to top in the following order, on the top of the other, the lower pressing plate 11a, the lower separating sheet 12a, the set of laminated sheets 7a, the intermediate separating sheet 12b, the other set of laminated sheets 7b, the upper separating sheet 12c and the upper pressing plate 11b. A plurality of through-holes 17, 18 are provided at prescribed points on the thus prepared block, and mutual slip between the components of the block is prevented by inserting pins (not shown) into these through-holes 17, 18.
The above-mentioned block is mounted on the lower pressing portion 15 equipped with a heater, provided on the stand 14 of the hot press 13. The hot press 13 is actuated in this state to cause the upper pressing portion 16 equipped with a heater to go down and thus to apply a pressure onto the block at a prescribed temperature. As a result, the epoxy resin impregnated in each of the prepreg 9a and the other prepreg 9b is melted, and this causes the sheet of copper foil 10a and the other sheet of copper foil 10b to be respectively bonded onto the both surfaces of the synthetic resin sheet 8 in the set of laminated sheets 7a, thus forming a printed circuit board. A sheet of copper foil and another sheet of copper foil are also respectively bonded onto the both surfaces of a synthetic resin sheet in the other set of laminated sheets 7b in the same manner as described above, thus forming another printed circuit board.
After the completion of bonding of the sheet of copper foil 10a and the other sheet of copper foil 10b onto the both surfaces of the synthetic resin sheet 8, the hot press 13 is actuated to cause the upper pressing portion 16 to go up to release the pressure on the block, and thus to take out the block from the hot press 13. Then, the above-mentioned pins are removed from the through-holes 17, 18 of the block thus taken out, and the lower pressing plate 11a, the lower separating sheet 12a, the intermediate separating sheet 12b the upper separating sheet 12c and the upper pressing plate 11b are removed from the block, thereby, simultaneously manufacturing a plurality of printed circuit boards.
The above-mentioned method for simultaneously manufacturing the plurality of printed circuit boards has the following problems: When applying a pressure onto the block by the use of the hot press 13 as described above, part of the melted epoxy resin between the synthetic resin sheet 8, the sheet of copper foil 10a and the other sheet of copper foil 10b is forced out from the edges of these sheets and from the through-holes 17, 18 inserted with the above-mentioned pins, and adheres onto the surfaces of the lower pressing plate 11a, the lower separating sheet 12a, the intermediate separating sheet 12b, the upper separating sheet 12c, and the upper pressing plate 11b.
Adhesion of the epoxy resin onto the surfaces of the lower pressing plate 11a, the lower separating sheet 12a, the intermediate separating sheet 12b, the upper separating sheet 12c and the upper pressing plate 11b causes the following demerits:
(1) It becomes difficult to pull out the pins from the through-holes 17, 18 of the above-mentioned block; PA1 (2) It becomes difficult to remove the lower pressing plate 11a, the lower separating sheet 12a, the intermediate separating sheet 12b, the upper separating sheet 12c and the upper pressing plate 11b from the block; PA1 (3) When applying a pressure onto another block by means of the hot press 13 in the same manner as described above by the use of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c, onto the surfaces of which the epoxy resin adheres, flaws are caused on the surface of the copper foil of the resultant printed circuit board by the epoxy resin adhering onto the surfaces of these separating sheets 12a, 12b and 12c. PA1 (4) It is therefore necessary, after the use of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c, to always remove the epoxy resin adhering onto the surfaces of these separating sheets 12a, 12b and 12c by means of a metal spatula or a knife; PA1 (5) However, the above-mentioned removing operation of the epoxy resin tends to cause flaws on the surfaces of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c; PA1 (6) When applying a pressure onto another block by means of the hot press 13 in the same manner as described above by the use of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c, on the surfaces of which the above-mentioned flaws are produced, flaws are caused on the surface of the copper foil of the resultant printed circuit board by the flaws produced on the surfaces of these separating sheets 12a, 12b and 12c; and PA1 (7) It is therefore necessary to polish frequently the surfaces of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c to made smooth. As a result, these separating sheets 12a, 12b and 12c become thinner and the service lives thereof are reduced. PA1 (A) Each time a pressure is applied onto the block by the use of the hot press 13, it is necessary to replace the used fluorocarbon polymer film with a new one, this resulting in more complicated operations and leading to a higher manufacturing cost; and PA1 (B) Even by the use of the above-mentioned fluorocarbon polymer film, it is impossible to completely prevent adhesion of the epoxy resin onto the surfaces of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c. More specifically, it is necessary to form holes for inserting the above-mentioned pins also in the fluorocarbon polymer film, and as a result, the epoxy resin flows through the holes in the fluorocarbon polymer film, and the through-holes 17, 18 in these separating sheets 12a, 12b and 12c, and adheres onto the surfaces thereof. PA1 (a) The plating layer on the inner surface of the mold of the prior art 2 may comprise a nickel-phosphorus alloy; and PA1 (b) A fluorocarbon polymer layer may be formed, on the surface of the above-mentioned plating layer, by melting the fluorocarbon polymer particles which are exposed on the surface of the plating layer.
As a means to solve the above-mentioned problems, there is known a method for preventing adhesion of an epoxy resin onto the surfaces of the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c by covering the surfaces of these separating sheets 12a, 12b and 12c with a fluorocarbon polymer film (hereinafter referred to as the "prior art 1").
However, the prior art 1 has the following defects:
Japanese Utility Model Publication No. 60-10,816 dated Apr. 12, 1985 discloses a mold for forming plastics, rubber or glass, which has on the inner surface thereof a plating layer in which fluorocarbon polymer particles are uniformly dispersed (hereinafter referred to as the "prior art 2").
The prior art 2 further discloses the following:
The lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c, which are used when applying the pressure onto the block by the use of the above-mentioned hot press 13, are required to have a Vickers hardness of at least 500 Hv. In the prior art 2, however, no regard is given to the effect of both the phosphorus content in the plating layer and the content of the fluorocarbon polymer particles in the plating layer, exerted on both hardness and strippability of the plating layer, in the case where the plating layer comprises a nickel-phosphorus alloy. In the prior art 2, as a result, it is not ensured that the plating layer is excellent in strippability and has a Vickers hardness of at least 500 Hv.
It is not therefore possible to apply the technical idea of the prior art 2 to the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c, which are used when applying the pressure onto the block by the use of the hot press 13. Even when the technical idea of the prior art 2 is applied to the lower separating sheet 12a, the intermediate separating sheet 12b and the upper separating sheet 12c to form a plating layer, in which the fluorocarbon polymer particles are uniformly dispersed, on the surfaces of these separating sheets 12a, 12b and 12c, it is not ensured that the thus obtained separating sheets simultaneously satisfy the following two conditions: (1) The separating sheets are excellent in strippability, i.e., they have a strippability sufficient to permit easy removal of the epoxy resin adhering onto the surfaces thereof by the use of a metal spatula or a knife; and (2) The separating sheets have a sufficient hardness, i.e., a Vickers hardness of at least 500 Hv.
Therefore, if the thus obtained separating sheets are used when applying the pressure onto the block by the use of the hot press 13, the following problems may be caused: (a) It is difficult to completely remove the epoxy resin adhering onto the surfaces of the separating sheets even with the use of a metal spatula or a knife; and (b) During the above-mentioned removing operation of the epoxy resin, the metal spatura or the knife may cause deep flaws in the plating layer, and moreover, may cause flaws, not only in the plating layer, but also in the substrate itself, and such flaws in the plating layer and the substrate degrade the quality of the printed circuit board, as described above.
Under such circumstances, there is a strong demand for the development of a plated metal sheet provided with a plurality of plating layers, excellent in strippability and having a high hardness, which is most suitable as a separating sheet to be used when manufacturing a printed circuit board by the use of a hot press, but such a plated metal sheet has not as yet been proposed.